Aircraft include one or more movable flight control members such as ailerons on the wings for roll control, elevators on the horizontal tail of the empennage for pitch control, a rudder on the vertical tail of the empennage for yaw control, a nose cone of a missile, and other movable control surfaces. Movement of a flight control member is typically effected by one or more actuators mechanically coupled between a base (e.g., a wing spar, a platform of a missile) and the flight control surface (e.g., an aileron, nose cone). In many aircraft, the actuators for flight control members are linear hydraulic actuators driven by one or more hydraulic systems.
A challenge with aircraft is preventing the occurrence of flutter of the flight control members during flight. Control member flutter can be described as unstable aerodynamically-induced oscillations of the flight control member, and can occur in flight control systems where the operating bandwidth of the flight control system overlaps the resonant frequency of the flight control surface. Unless damped, the oscillations can rapidly increase in amplitude with the potential for undesirable results, including exceeding the strength capability of the mounting system of the flight control member and the actuator. Contributing to the potential for control member flutter is elasticity in the flight control system. For example, hydraulic actuators can exhibit a linear spring response under load due to compressibility of the hydraulic fluid. The compressibility of the hydraulic fluid can be characterized by the cross-sectional area of the actuator piston, the volume of the hydraulic fluid, and the effective bulk modulus of elasticity of the hydraulic fluid.